Although such etching may be of use in various fields, its effective application is limited at present to certain types of etching when fabricating semiconductor components based on indium phosphide.
The present invention is applicable, more particularly, to fabricating optoelectronic components for use at wavelengths of 1.3 micrometers or greater. Such components are used in telecommunications. Requirements in this field are tending to components which are more and more complex and which are made from wafers of monocrystaline indium phosphide InP. Their manufacture requires a plurality of etching operations in this material.
During each of these operations, an etching procedure is implemented to etch a face which has previously been partially protected by a mask. This etching causes an interconnection flank to appear all around the edge of the mask between the intact zone which was protected by the mask and the zone which was eaten away by the etching. The profile of this flank depends on the etching method selected and on the orientation of the edge under consideration. The higher intact zone constitutes a "mesa" whose "type" may be defined by said profile.
Depending on the desired type of profile, several known etching methods may be used. At present, the etching method used most widely in industry is a wet method based on a bromine-methanol solution: a wafer 200 (see FIG. 1) is used whose top face to be etched is oriented on (001) plane. When the protective mask is a tape aligned with the (110) direction, a mesa 202 of the so-called "inverted" type is obtained. The (111) plane is not etched, where the symbol 1 represents the digit 1 with a bar on top; however, etching with a mask aligned with the (110) direction gives rise to a mesa 204 of the so-called "normal" type. The etching profile is very sensitive to crystal planes of any orientation: the flanks of the resulting mesa are substantially plane and are at an angle with the top face of the mesa which is acute in the first case and which is obtuse in the second case.
Other etching solutions, e.g. dilute hydrochlorid acid, may be used when performing wet etching. However these tend to be laboratory methods which are difficult to industrialize.
InP can be dry etched by reactive ion etching (RIE) using a plasma made up of chloride gases which are polarized relative to the wafer to be etched. This method gives rise to etching profiles which are completely insensitive to crystal planes (mesas having vertical flanks).
These various known methods suffer from the drawback of sometimes not enabling a desired profile to be obtained, in particular when a first strip mesa extending in a longitudinal direction has been made and is of the inverted type, and when a second strip mesa running along the same longitudinal direction is to be made but is to be of the normal type. This occurs when the flanks of the second mesa are to receive an effective passivation layer.
Such a layer cannot be obtained by deposition onto the flanks of an inverted mesa. It is very difficult to obtain on vertical flanks.
The object of the present invention is to make it possible, in some cases, to obtain a desired mesa profile which conventional etching methods are incapable of providing.
Another object of the invention is to enable effective passivation on the flanks of a strip-shaped mesa whose orientation on a monocrystaline semiconductor wafer is imposed by other considerations.
Another object of the invention is to make it possible to obtain a highly resistive surface layer on an etched surface of a monocrystaline part.
Yet another object of the invention is to make etching possible using substances which are not very dangerous, which are cheap, and which are easy to manipulate.